60 Instinct and Intelligence 



intimately connected with it, a plexus of nerve 

 cells and fibres exists beneath the ectoderm, 

 corresponding to that found on the under sur- 

 face of the swimming-bell of Medusoids. 1 



The sensory organs of a star-fish are derived 

 from the ectoderm, and consist of an abundance 

 of tactile organs situated on the spines and 

 pedicels of the animal's arms; in some species 

 of star-fish a bright red spot exists on the ter- 

 minal tentacles of each of the five arms, which 

 indicates the position of the animal's visual 

 sensory organs, these are of a higher order 

 than those we have referred to as the eye-spots 

 of unicellular beings and o'f Medusoids. The 

 outer surface of the visual organs of Echino- 

 derms is formed of one or more layers of trans- 

 parent cells, and beneath this layer a refractive 

 lens-like structure rests on a mass of deeply 

 pigmented cells. In connection with these 

 colour-bearing cells, club-shaped nucleated 

 cells may be demonstrated which receive at 

 their base fibres from terminal branches of a 

 radial nerve; in this way a communication 

 is established between the animal's visual 



1 Romanes on Jelly-fish, p. 292. 



